Pulmonary infections caused by the group of nontuberculosis mycobacteria (NTM), Mycobacterium avium complex (MAC), are a growing public health concern with incidence and mortality steadily increasing globally. Granulomatous inflammation is the hallmark of MAC lung infection, yet reliable correlates of disease progression, susceptibility, and resolution are poorly defined. Unlike widely used inbred mouse strains, mice that carry the mutant allele at the genetic locus sst1 develop human-like pulmonary tuberculosis featuring well-organized caseating granulomas. We characterized pulmonary temporospatial outcomes of intranasal and left intrabronchial M. avium spp. hominissuis (M.av) induced pneumonia in B6.Sst1S mice, which carries the sst1 mutant allele. We utilized traditional semi-quantitative histomorphological evaluation, in combination with fluorescent multiplex immunohistochemistry (fmIHC), whole slide imaging, and quantitative digital image analysis. Followingintrabronchiolar infection with the laboratory M.av strain 101, the B6.Sst1S pulmonary lesions progressed 12–16 weeks post infection (wpi), with plateauing and/or resolving disease by 21 wpi. Caseating granulomas were not observed during the study. Disease progression from 12–16 wpi was associated with increased acid-fast bacilli, area of secondary granulomatous pneumonia lesions, and Arg1+ and double positive iNOS+/Arg1+ macrophages. Compared to B6 WT, at 16 wpi, B6.Sst1S lungs exhibited an increased area of acid-fast bacilli, larger secondary lesions with greater Arg1+ and double positive iNOS+/Arg1+ macrophages, and reduced T cell density. This morphomolecular analysis of histologic correlates of disease progression in B6.Sst1S could serve as a platform for assessment of medical countermeasures against NTM infection.

Pulmonary infections caused by the group of nontuberculosis mycobacteria (NTM), Mycobacterium avium complex (MAC), are increasing worldwide and a growing public health concern. Pulmonary granulomas are the hallmark of MAC lung infection, yet reliable correlates of granuloma progression and susceptibility in immunocompetent hosts are poorly defined. The development of mouse models that recapitulate the diversity of granulomas seen in MAC pulmonary disease in humans is crucial to study mechanisms of susceptibility in humans and for preclinical evaluation of therapeutics. Unlike widely used inbred mouse strains, mice that carry the mutant allele at the genetic locus sst1 develop human-like pulmonary tuberculosis featuring well-organized caseating granulomas. These mice became instrumental in pre-clinical testing of novel interventions. In this study, we tested whether the B6.Sst1S that carries the sst1 mutant allele on standard B6 background develop more advanced pulmonary infection with NTM M. avium spp. hominissuis (M.av). To assess pulmonary disease progression, we utilized traditional semi-quantitative histomorphological evaluation and fluorescent multiplex immunohistochemistry (fmIHC) in combination with whole slide imaging and digital image analysis. After infection with the laboratory M.av strain 101, the B6.Sst1S pulmonary lesions progressed 12 to 20 weeks post infection, although we did not observe the formation of necrotic granulomas during this interval. Using fmIHC, we determined that the disease progression was associated with a steadily increasing proportion of mycobacteria infected Arg1+ and double positive iNOS+Arg1+ macrophages. The B6.Sst1S granulomas had a greater proportion of Arg1+ and double positive iNOS+Arg1+ macrophages, and decreased T cell density, as compared to wild type B6 mice. Thus, the genetic composition of the B6.Sst1S mice renders them more susceptible to pulmonary M.av infection. In combination with more virulent clinical isolates of M.av these mice could provide an improved mouse model that recapitulates more severe pulmonary disease in humans. The Arg1 macrophage expression in this model combined with automated fmIHC could serve as a sensitive biomarker for the unbiased assessment of medical countermeasures against NTM infection.